glossary: explain "master" and "origin"

If you are a long time git user/developer, you forget that to a new git
user, these words have not the same meaning as to you.

[jc: with updates from J. Bruce Fields.]

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>